I have edited your question to reflect what I think you were asking. If that is not what you were asking, feel free to let me know.
There are many different theories as to what causes imperialism. For example:
- Marxists believe that imperialism comes naturally out of capitalism. They say that capitalists need markets and sources of raw materials and that they control the governments. Therefore, the governments go out and conquer empires to give the capitalists what they need.
- Political/realist theories. These theories hold that imperialism happens because countries need power. Countries are, in this view, always seeking power. They believe that taking an empire will be a good way to strengthen themselves relative to possible rivals.
Follow this link for more theories about the causes of imperialism.